Overview of SATA Protocol
A “device” as used herein refers to a peripheral adhering to any known standard adopted by the industry. SATA is a high-speed serial link replacement for the parallel Advanced Technology Attachment (ATA) attachment of mass storage devices. The serial link employed is a point-to-point high-speed differential link that utilizes gigabit technology and 8b/10b encoding known to those of ordinary skill in the art. The SATA protocol is based on a layered communication model similar to Open Systems Interconnection (OSI) Reference Model. An overview is presented below. For more detail, the reader is referred to the SATA standard incorporated herein by reference. The SATA specification is provided in the publication entitled “Serial ATA: High Speed Serialized ATA Attachment” Revisions 1.0, dated Aug. 29, 2001, and the publication entitled “Serial ATA II: Extensions to Serial ATA 1.0”, Revision 1.0, dated Oct. 16, 2002, both of which are currently available at Serial ATA work group web site www.serialata.com.
In the SATA protocol, each layer of protocol communicates with its counterpart directly or indirectly. FIG. 1a shows the SATA protocol communication layers 20. The Physical (Phy) layer (PL) 21 manages the physical communication between the SATA units. The services of PL include:                serializing a parallel input from the link layer (LL) 22 and transmitting differential Non-Return to Zero (NRZ) serial stream.        receiving differential NRZ serial stream, extracting data (and optionally, the clock) from the serial bit stream, deserializing the serial stream, and providing a bit and a word aligned parallel output to the LL 22        performing the power-on sequencing, and performing speed negotiation,        providing specified out of band (OOB) signal detection and generation        
The serial ATA link is defined by a protocol pursuant to a known standard, having four layers of communications, the physical layer for performing communication at a physical level, a link layer, a transport layer and an application layer or sometimes referred thereto as a command layer. A transmitter and a receiver, cannot directly communicate the latter with each other, rather, they must go through the other layers of their system prior to reaching a corresponding layer of the other. For example, for the physical layer of a transmitter to communicate with the transport layer of the receiver, it must first go through the link, transport and application layers of the transmitter and then through the serial ATA link to the application layer of the receiver and finally to the transport layer of the receiver.
The basic unit of communication or exchange is a frame. A frame comprises of a start of frame (SOF) primitive, a frame information structure (FIS), a Cyclic Redundancy Checksum (CRC) calculated over the contents of the FIS and an end of frame (EOF) primitive. The serial ATA organization has defined a specification in which the definition of a frame is provided and which is intended to be used throughout this document. Primitives are double word (Dword) entities that are used to control and provide status of the serial line. The serial ATA organization has defined a specification in which the definition of allowed Primitives is provided and which is intended to be used throughout this document
FIG. 1b shows an example of a frame 30. The frame, in FIG. 1b, starts with an SOF primitive 30a, followed by a first FIS content 30b, followed by a HOLD primitive 30c indicating that the transmitter does not have data available, followed by a second FIS content 30d, followed by a HOLDA primitive 30e sent to acknowledge receipt of HOLD primitive, sent by the receiver, indicating that the receiver buffer is in a ‘not ready’ condition, followed by a CRC 30f and an EOF primitive 30g. 
The frame, in FIG. 1b, includes two primitives a HOLD and a HOLDA primitive used for flow control. A HOLD primitive indicates inability to send or to receive FIS contents. A HOLDA primitive is sent to acknowledge receipt of a HOLD primitive. For example, when a receiving node detects that its buffer is almost full, it will send a HOLD primitive to a transmitting node, requesting the transmitter node to stop and when the buffer is ready to receive more data, the receiving node will stop sending a HOLD primitive. The transmitting node sends a HOLDA primitive to acknowledge receipt of the HOLD primitive. Until receipt of the HOLDA primitive, the receiving node continues receiving data. In order to prevent a buffer overrun, the SATA protocol requires a maximum delay of 20 Dwords between a node sending the HOLD primitive and receiving a HOLDA primitive.
There are a number of different frame types, as shown in FIG. 1d. For example, to send data via Direct Memory Access (DMA), a frame known as DMA setup FIS is utilized followed by a DMA data FIS. There are generally three types of FIS structures, one for commands, one for setting up a transfer and another for data relating to the transfer. Each frame structure is used for a different purpose. A command type of frame is sent to execute a command, a setup frame is used to prepare for the data transfer phase of the command and a data frame is used to transfer data. At the command layer, the system communicates with the command layer through the task file, mentioned hereinabove and shown in FIG. 1c. The command layer uses two distinct busses for communication, one is for transferring data FIS and the other is for transferring non-data FIS. Although 2 busses are discussed herein in a single bus may be employed.
The link layer (LL) 22 transmits and receives frames, transmits primitives based on control signals from the PL 21, and receives primitives from Phy layer (PL) 21 which are converted to control signals to the transport layer (TL) 23.
The transport layer (TL) 23 need not be cognizant of how frames are transmitted and received. The TL 23 simply constructs frame information structures (FIS's) for transmission and decomposes the received FIS's.
FIG. 1d shows the FIS types. The FIS types are summarized below:                Register FIS—host to device 40(i)        Register FIS—device to host 40(ii)        DMA Activate FIS 40(iii)        DMA Setup FIS 40(iv)        Set Device Bits FIS 40(v)        PIO Setup FIS 40(vi)        Data FIS 40(vii)        BIST Activate FIS 40(viii)        
In the application layer of the serial ATA link, the host accesses a set of registers that are ATA registers, data port, error, features, sectors, cylinder low, cylinder high, status and command. Thus, the application layer communicates in the same language as the ATA standard, which is at the command layer. Thus, the command layer uses the same register set as the ATA link. The register set is known as task file registers.
The command layer (CL) or application layer (AL) 24 interacts with TL 23 for sending/receiving command, data, and status. The CL 24 includes register block register; also known as a task file (TF), used for delivering commands or posting status that is equivalent to that provided by a traditional parallel ATA.
FIG. 1c shows a simplified version of the Shadow Register Block organization 31 of parallel ATA. The Shadow Register Block comprises                Data Port 31dp         Error Register 31e         Features Register 31f         Sector Count 31sc         Sector Number 31sn         Cylinder Low 31cl         Cylinder High 31ch         Device/Head 31dev         Status 31s         Command 31c         Alternate Status 31as         Device Control 31dc         
A SATA port, including part or all of the layer 1 functions, will be referred to herein as the SATA level 1 port. A SATA port, including part or all of the layers 1 and 2 functions, will be referred to herein as a SATA level 2 port. A SATA port, including part or all of the layers 1, 2, and 3 functions, will be referred to as a SATA level 3 port. A SATA port, including part or all of the layers 1, 2, 3 and 4 functions, will be referred to herein as a SATA level 4 port. The term SATA port refers to a generic port including level 1 or level 2 or level 3 or level 4. The SATA layers are for coupling to either the host or the device. The term SATA host port refers to a SATA port connected to a host. The term SATA device port refers to a SATA port connected to a device. For example, if the outbound high speed differential transmit signals 51tx and the inbound differential receive signals 51rx of FIG. 2a are connected to a host, the SATA port is a SATA host port. Similarly, if the outbound high speed differential transmit signals 51tx and the inbound differential receive signals 51rx of FIG. 2a are connected to a device, the SATA port is a SATA device port.
FIGS. 2a and 2b show block diagrams of a SATA port 50. The SATA port 50 includes a PL circuit 51, a LL circuit 52, a TL circuit 53 and a CL circuit 54. The PL circuit 51 is connected to outbound high speed differential transmit signals 51tx and inbound differential receive signals 51rx, the PL circuit 51 is connected to the LL circuit 52 via a link transmit bus 52t, and a link receive bus 52r. The PL circuit 51 comprises an analog front end (AFE) 51a, a phy initialization state machine (Phy ISM) 51b, an out-of-band (OOB) detector 51c, a Phy/Link Interface 51e. The Phy/Link interface block optionally includes an elastic first-in-first-out (FIFO) 51ef and a transmit FIFO 51tf. The Phy/Link Interface 51e provides the coupling of the PL circuit 51 to the LL circuit 52 via the link transmit bus 52t, and the link receive bus 52r. A multiplexer 51d, controlled by the Phy ISM 51b, selects the link transmit data 51t or the initialization sequence 51s from the Phy ISM 51b. The AFE 51a includes the Phy receiver and Phy transmitter. The AFE 51a couples differential transmit signals 51tx and differential receive signals 51rx to the receive data 51r and to the transmit data 51td. The Phy transmitter is enabled by the Phy Transmitter Enable (PhyTxEn) signal 51te. When the Phy transmitter is disabled, the Phy output is in the idle bus state (Tx differential signal diminishes to zero). The OOB detector 51c detects out of band (OOB) signals 51o. The OOB signals 51o comprise COMRESET, COMWAKE.
The LL circuit 52 is connected to the PL circuit 51 via the link transmit bus 52t and the link receive bus 52r. The LL circuit 52 is connected to the TL circuit 53 via a transport transmit bus 53t, a transport receive bus 53r and a transport control/status bus 53c. The TL circuit 53 comprises a data FIS First-In-First-Out (FIFO) circuit 53a for holding the data FIS during transit, a block of non-Data FIS Registers 53b for holding non-Data FIS, and a multiplexer 53d. The data FIS FIFO 53a is a dual port FIFO, each port having separate input and output. The FIFO 53a comprises a first FIFO port 53a(1) and a second port 53a(2), the first port further including a first input port 53a(i1) and a first FIFO output port 53a(o1), the second port further including a second FIFO input port 53a(i2), and a second output port 53a(o2).
The first FIFO port 53a(1) is coupled to the LL circuit 52 via the said transport transmit bus 53t, the transport receive bus 53r and the transport control/status bus 53c. The second FIFO port 53a(2) is coupled to the CL circuit 54 via the data FIS receive bus 54r and the data FIS transmit bus 54t. The TL circuit 53 is coupled to the CL circuit 54 via a task file input bus 54i and a task file output bus 54o. The multiplexer 53d selects between the first FIFO output port 53a(o1) and the task file input bus 54i. The CL circuit 54 comprises a Task File 54a. The Task file 54a is coupled to the TL circuit 53 via the task file input bus 54i and the task file output bus 54o. The Task file 54a is coupled to the system bus 57 via the port task file input bus 56i and port task file output bus 56o, the CL circuit 54 additionally couples the Data FIS receive bus 54r and the Data FIS transmit bus 54t to system bus 57 via a data input bus 55i and the data output bus 55o. A configuration signal configures the operation of the SATA port for host or device operation The CL circuit 54 may be coupled to the system bus 57 via a single bus for data port and task file access.
The SATA switches of prior art allow two different hosts to connect to the same device, however, when one host is connected to the device, the other host can not access the device. Such limitations of prior art systems will be further explained. The SATA switches of prior art do not allow two hosts to access the device concurrently.
FIG. 3a shows a system 10 using a prior art SATA switch 14. The system 10 is shown to include a host 11 coupled to a SATA Host Bus Adaptor (SATA HBA) 11a, the SATA HBA 11a is shown to be coupled to a host port 14a of the SATA switch 14 via a SATA link 11b and a host 12, which is shown coupled to a SATA HBA 12a, which is shown coupled to a host port 14b of the SATA switch 14 via a SATA link 12b. The device port 14c of the SATA switch 14 is shown coupled to a storage unit 16, such as a hard disk drive (HDD) or a Tape Drive or Optical Drive via a SATA link 16a. The storage unit 16 is an example of a device.
A select signal 15 selects either the host port 14a or the host port 14b of the SATA switch 14. The port that is coupled to the currently-selected host on the SATA switch is considered an active port whereas the port that is not coupled to the currently-selected host is considered the inactive port. An active host as used herein indicates a host that is currently being selected.
Two methods are used to select the active port, side-band port selection and protocol-based port selection. In the side-band port selection method, the SATA switch 14 operatively couples either the host 11 or the host 12 to the device 16 based on the state of the select signal 15. The mechanism for generating the select signal 15 is system dependent. The protocol-based port selection uses SATA protocol on the inactive host port to cause a switch to activate. The protocol-based port selection uses a sequence of SATA OOB signals to select the active port. The aforementioned methods only allow access to a storage unit by a single host at any given time. This type of SATA switch is referred to as a simple failover switch.
FIG. 3b shows a system application of the SATA to ATA switch 64. The SATA to ATA Switch 64 comprises of a SATA port 64a coupled to a host 11, a SATA port 64b coupled to a host 12 and an ATA port 64c coupled to a storage unit 66. In system 60, the storage unit 66 has an ATA link and the ATA port 64c is coupled to a storage unit 66 via an ATA link 66a. 
The use of the simple failover switch is in applications where in the event of failure of the primary host, the system switches to a standby secondary host, hence the name simple failover switch. In these types of systems, the operation of the system is interrupted and a “glitch” occurs. Obviously, mission-critical systems that cannot afford a failure, require uninterrupted system operation when a failure occurs. Mission-critical systems thus require concurrent access by both hosts to the storage unit, therefore, a mission critical system can not use a simple failover switch and instead uses dual-ported storage units, wherein the storage unit can be accessed concurrently from both ports. Fiber channel (FC) hard disk drives (HDDs) are typically dual-ported and are generally used in mission critical systems. FC HDDs are typically an order of magnitude more expensive than SATA HDDs. There is an economic need, however, to use the less expensive ATA or SATA HDDs in the storage units for mission-critical systems. However, ATA or SATA HDDs are single-ported and a simple failover switch does not allow concurrent access to the storage unit by multiple hosts.
Therefore, there is a need for electronic switches allowing access by host to devices, such as storage units wherein concurrent access is allowed from two or more host ports to a single-ported storage unit connected to the device port of a switch via a SATA link or an ATA link.
The SATA switch will cause additional delays in the signal path that may cause failure to meet the timing requirement of the SATA protocol timing requirement for signal path. There is a need for a SATA switch wherein, with the additional delay of the switch, the timing requirements of the SATA protocol are met. “Host”, as used herein below, refers to either the host 11 or 12 of FIGS. 3a and 3b, depending on the context of the discussion. Similarly “device” as used herein below, refers to device 16 of FIGS. 3a, and 3b. 